Baltic Sea – threatened multi -talent | nd-aktuell.de

At the northwest tip of the Great Danish island of Funen, the world still seems okay: the green -brown leaves of the bladder tang (fucus vesiculosus) weigh themselves in the waves. On the sea floor, purple, pale pink and ocher-colored naval stars shine, crab searches for snails, and further outside, in the middle of the sea, swims of pork whales.

Like seaweed meadows, bladder tangwals are one of the typical ecosystems of the Baltic Sea. They accommodate up to 500 species, from unicellulars to fish that spawn on them or hide in them. They supply the algae with oxygen and clean the water. They also make an important contribution to climate protection, as a study of Bremen scientists published at the end of 2022 shows: Algae add up to a third of the carbon absorbed in the form of sugary compounds into the ambient water. Very few specialized bacteria are able to disassemble the multiple sugar Fucoidan retired by the bladder tang. “The more complex a sugar is, the slower it will be microbially decomposed, or it even stays in the long term,” explains Inga Hellige, who worked on the study and has now worked as a postdoctorant at the University of Gothenburg. So the carbon would remain bound for centuries or even thousands of years.

On the south coast of Fünens there is a completely different picture than in the northwest of the island: thick upholstery piled up on the small beaches, already dried thread algae. Large brown algae carpets drive off the coast. Even in the far southwest, the waves carry algae on land, it stinks of lazy eggs – a smell that arises when it is decomposed.

Bladdering forests accommodate up to 500 species, from individuals to fishing.

In fact, there has been strong oxygen shortage since the end of August in the Inselmeer in the southern Sea and the South of the small Belt, as in Aabenraa Fjord, Schlei, Flensburg Fjord, Fehmarn Belt and Aarhus. This is demonstrated by a recently published report by the Danish Center for Environment and Energy (DCE) at the University of Aarhus. Fish were found inside the Haderslev Fjord at the end of July. Although the oxygen content of Danish waters has reached record lows for the past two years, the situation has worsened. “The affected area was (in July and August 2025) a fifth larger than 2024 and 3.5 times the size of 2023,” summarizes Christian Fromberg from Greenpeace Denmark. The peak is usually only reached in September. “If oxygen poverty continues as expected, this will have serious consequences, especially for life on the sea floor in the most affected areas,” warn the author of the report.

This is due to the high nutrient supply from agriculture. “This leads to an explosive growth of plankton and opportunistic thread algae in summer,” said Fromberg. When decomposing, microbes consume large amounts of oxygen. The thread algae, according to the Andersen from the Institute for Eckey Sciences at the University of Aarhus, are the brown algae Pylaiella Littoralis and EctoCarpus penicillatus as well as some types of green and red algae. »These brown thread algae grow like Fucus SPP. (The family belongs to that of the bladder tang, note of the A.) on hard surfaces such as rocks or stones, and as upholstery plants on seagras and bladderang, «explains the research assistant for marine protection at the Federal Environment Agency, Stephanie Helber. “If you multiply through high nutrient entries, you literally overgrow seaweed and bladdered.”

But the large Braun algae is “quite robust,” assures the Kiel marine ecologist Martin Wahl. While the thread algae are one year and die in late summer, the multi -year bladderang could recover despite little light at low temperatures in winter. The thread algae could still become a problem if they occur in very large quantities, warns the marine expert from the federal government for the environment and nature conservation (federal government), Dorothea Seeger. They are torn from their hard substrate in large storms and drive around freely in the sea. Some of them are thrown by the waves to the bank, another rotted in the water and, in extreme cases, consumes the entire oxygen supply on the seabed.

The bladder tang in the Baltic Sea has decreased massively since the 1960s. “In the past, he came to a depth of eight to ten meters because the water was clearer,” says Wahl. Today it can only be found in large parts of the Baltic Sea in the first three meters. The rising water temperatures also cause him.

In order for it to be spread out again, strict requirements for agriculture, as already decided and implemented Sweden in the 1960s, are needed. This year’s study of Swedish scientists under the direction of Ellen Schagerström shows that the bladder tang stocks have recovered north of Stockholm.

In Denmark, the nutrient entries are still too high in the past 40 years despite various government measures. A new plan named »Tripart« primarily targets animal husbandry, but only comes into force from 2027. According to the UBA, the situation in Germany has improved since 1990, but there is still a lot of room for improvement here.

In the still ongoing research project “Clipa”, scientists from the universities of Kiel and Rostock and the Geomar examine whether bladder tang can be cultivated in offshore wind turbines. To do this, you have stretched ropes directly under the water surface. The algae should filter nutrients from the water and bind carbon. It can be harvested all year round and are used energetically as a pasta replacement, for ointments and in biogas plants. The fermentation residues were an inexpensive and environmentally friendly alternative to the artificial fertilizer. The problem of mass infestation with mussel larvae is still unresolved, reports Wahl, who works in the project. “However, recent results show that young algae survive for a very long time under the shell cover and apparently grow after their disappearance.”